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May 26, 2017 - Determination and Correlation of Ethyl Vanillin Solubility in Different. Binary Solvents at Temperatures from 273.15 to 313.15 K. Nanna...
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Determination and Correlation of Ethyl Vanillin Solubility in Different Binary Solvents at Temperatures from 273.15 to 313.15 K Nannan Guo,† Baohong Hou,†,‡ Hao Wu,† Jingjing Huang,† Xiaolong Tao,† Xin Huang,†,‡ Qiuxiang Yin,†,‡ and Hongxun Hao*,†,‡ †

State Key Laboratory of Chemical Engineering, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, People’s Republic of China ‡ Collaborative Innovation Center of Chemical Science and Chemical Engineering (Tianjin), Tianjin 300072, People’s Republic of China S Supporting Information *

ABSTRACT: The solubility data of ethyl vanillin in binary solvents, including (propan-2-one + water), (methanol + water), (ethanol + water), and (propan-2-ol + water), were measured in the temperature range from 273.15 to 313.15 K by using a UV spectroscopy method. The results show that the solubility of ethyl vanillin increases with increasing temperature. Besides, the modified Apelblat equation and the λh equation were used to correlate the experimental solubility data. Dissolution thermodynamic properties of ethyl vanillin in different binary solvent mixtures, including Gibbs energy, enthalpy, and entropy, were calculated based on the nonrandom two-liquid equation and the experimental solubility data.

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to optimize this process.6 Therefore, it is necessary to obtain accurate solubility data of ethyl vanillin in different solvents. In the crystallization of ethyl vanillin, organic solvents, such as propan-2-one, methanol, ethanol, and propan-2-ol, can be used as solvents while water can be used as antisolvent. Therefore, in this work, the solubility data of ethyl vanillin in four binary solvents (water + propan-2-one/methanol/ethanol/ propan-2-ol) were measured with a UV spectroscopy method in the temperature range from 273.15 to 313.15 K. The modified Apelblat equation and the λh equation were used to correlate the experimental solubility data of ethyl vanillin. Besides, the dissolution thermodynamic properties, including the enthalpy, the Gibbs energy, and the entropy, were also calculated according to experimental solubility and nonrandom two liquid (NRTL) equations.

INTRODUCTION Ethyl vanillin (chemical name 3-ethoxy-4-hydroxybenzaldehyde, CAS Registry No: 121-32-4, C9H10O3) has a molar mass of 166.17 g/mol. With a strong vanilla bean flavor, the aroma of ethyl vanillin is about 3−4 times stronger than vanillin’s, and it retains a better condition in storage and transport.1,2 Therefore, ethyl vanillin is a good substitute for vanillin. Except for food additives, it is also widely used in spices, cosmetics, pharmaceuticals, and other industries involved with the roles of aroma enhancement function and perfume fixative agent.3 The chemical structure of ethyl vanillin is shown in Figure 1.

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EXPERIMENTAL SECTION Materials. Ethyl vanillin was supplied by Guangda Pharmaceutical Co. Ltd. (Beijing, China). Protocatechuic acid was supplied by Bide Pharmaceutical Co. Ltd. (Shanghai, China). Organic solvents including propan-2-one, methanol, ethanol, and propan-2-ol were analytical grade reagents and used without further processing. Distilled−deionized water was purchased from Nankai University, China. More information about the general properties of chemicals used in this work is listed in Table 1.

Figure 1. Chemical structure of ethyl vanillin.

Previous studies of ethyl vanillin mainly concentrated on its synthesis.4,5 Reports on the crystallization and purification technology of ethyl vanillin are rare. In the manufacturing process of ethyl vanillin, as a separation process, crystallization is a vital step to obtain solid products. To obtain product with high purity and yields, we need accurate equilibrium solubility © XXXX American Chemical Society

Received: November 20, 2016 Accepted: May 17, 2017

A

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Table 1. Description of Materials Used in This Papera chemical name

CASRN

source

mass fraction purity

purification method

analysis method

ethyl vanillin protocatechuic acid methanol ethanol propan-2-ol propan-2-one

121-32-4 99-50-3 67-56-1 64-17-5 67-63-0 67-64-1

Guangda Pharmaceutical Co. Ltd. Bide Pharmaceutical Co. Ltd. Tianjin Jiangtian Chemical Tianjin Jiangtian Chemical Tianjin Jiangtian Chemical Tianjin Jiangtian Chemical

≥0.990 ≥0.970 ≥0.995 ≥0.995 ≥0.995 ≥0.995

none none none none none none

HPLCb HPLCb GCc GCc GCc GCc

a Both the analysis method and the mass fraction purity were provided by the suppliers. bHigh-performance liquid chromatography. cGas chromatography.

X-ray Powder Diffraction. To identify the crystal form of ethyl vanillin, we used X-ray powder diffraction (XPRD) to test excess solid and dried samples in the temperature range from 273.15 to 313.15 K. The data were obtained on Rigaku D/max2500 (Rigaku, Japan) using Cu Ka radiation (0.15405 nm) through the 2 theta range from 2° to 50° with the scanning rate of 1 step per second. Melting Properties Measurements. The melting temperature and fusion enthalpy of ethyl vanillin were determined by using a Mettler-Toledo DSC 1/500 which was calibrated with indium and zinc standards before being analyzed. The measurements were within the temperature range from 298.15 to 373.15 K at a heating rate of 10 K·min−1. Nitrogen was used as protection gas (2.5 mL·s−1) in all measurements. Samples (0.005 to 0.010 g) of ethyl vanillin were weighed on an analytical balance (Mettler Toledo AB204-N, Switzerland) with an uncertainty of ±0.0001 g and then put into a sealed aluminum pan. The standard uncertainty for the melting temperature was 0.5 K, and the relative standard uncertainty for enthalpy of fusion was 0.05. Solubility Measurements by UV Absorption. Here we used the UV spectroscopic method to measure the concentration of ethyl vanillin in different binary solvent (water(A) + propan-2-one/methanol/ethanol/propan-2-ol(B)) mixtures. Moreover, the mole fraction of organic solvent (x0B) is from 0.40 to 1.00 at intervals of 0.10. The measurement process can be briefly described as follows. First, we added an excess amount of ethyl vanillin to stoppered conical flasks with a volume of 50 mL which were kept in a thermostatted shaker (Tianjin Ounuo Instrument Co. Ltd., China) which has the accuracy of ±0.1 K. After the flasks were continuously agitated for 12 h at a constant temperature, it can be confirmed that the (solid + liquid) equilibrium was reached, which has been proven in preliminary experiments. Then the agitation was stopped and the solid−liquid mixture was kept still for 5 h at the constant temperature to guarantee that the undissolved particles had settled. Next, we used preheated/cooled syringes and syringe filters (0.22 μm, Tianjin Legg Technology Co., Ltd., Tianjin, China) to take liquid samples from the upper clear saturated solution, respectively. Finally, the samples were diluted to a certain concentration which is appropriate for UV test on a UV-3010 spectrophotometer (HITACHI, Japan) with a 1 cm path length cell.7 The above process was repeated until three subsequent absorbance measurements were identical (within 5%), which was used to reduce the error. Moreover, to verify the reliability of the method, we measured the solubility of protocatechuic acid (PA) in methanol from (293.15 to 318.15) K and compared the obtained solubility data with literature data,8 as shown in Table S1 and Figure S1. It was found that the average relative error was less than 0.03.

Therefore, the experimental technique used in this work is reliable. The mole fraction solubility was described by eqs 1 and 2 based on the average absorbance value.9,10 mS =

A W ai

(1)

xS =

mS /MS mA /MA + mB /MB + mS /MS

(2)

where A is the absorbance value presented by UV spectrophotometer, ai is the fitting line slope, W is the diluted volume. mS represents the mass of ethyl vanillin; MS represent the molar mass of ethyl vanillin. mA and mB represent the mass of water and organic solvent (propan-2-one/methanol/ ethanol/propan-2-ol), respectively. MA and MB show the molar mass of water and organic solvent.. We prepared binary solvent mixtures according to following equations. mB =

mMBx B0 MBx B0 + MA xA0

(3)

m = mA + mB

(4)

where the meaning of mA, mB, MA, and MB are same as in eq 2; m represent the mass of solvent mixture. Besides, x0A and x0B represent the mole fraction of water and organic solvent. Before measuring the concentration of the saturated solution, we obtained the calibration curve of UV-concentration from an ethanol diluted solution of ethyl vanillin at room temperature with the maximum absorption wavelength (275.5 nm).

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THERMODYNAMIC MODELS Modified Apelblat Equation. The modified Apelblat equation, which is applied to correlate the solubility in (solid + liquid) equilibrium, is a semiempirical model deduced from the Clausius−Clapeyron model. Because of its simpleness, the modified Apelblat equation is used widely. It is shown as follows:11−13 ln xS = A +

B + C ln(T /K) T /K

(5)

where xS is the mole fraction solubility of ethyl vanillin, T is the absolute temperature of experiment, and A, B, and C are model parameters.14,15 λh Equation. In 1980, Buchowski put forward a new model, the λh model,16 which is an empirical equation. It can be used to describe the solubility of solute. B

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Table 2. Mole Fraction Solubility (xS) of Ethyl Vanillin in the Binary (Propan-2-one + Water) Solvent Mixtures at Different Temperatures from (273.15 to 313.15) K (p = 0.1 MPa)a x0B

102xexp S

0.40 0.50 0.60 0.70 0.80 0.90 1.00

10.6 13.4 16.8 18.5 19.5 19.8 19.9

0.40 0.50 0.60 0.70 0.80 0.90 1.00

12.8 16.2 19.2 21.0 21.4 21.5 21.6

0.40 0.50 0.60 0.70 0.80 0.90 1.00

13.7 18.8 20.9 23.1 23.4 23.5 23.7

0.40 0.50 0.60 0.70 0.80 0.90 1.00

17.3 20.7 23.8 26.4 26.7 26.9 27.1

0.40 0.50 0.60 0.70 0.80 0.90 1.00

20.5 25.2 26.9 29.1 29.6 29.8 30.0

102xcal,Apel S T = 273.15 K 10.6 13.7 16.9 18.7 19.4 19.6 19.7 T = 278.15 K 12.4 15.9 18.9 20.8 21.4 21.6 21.8 T = 283.15 K 14.5 18.4 21.1 23.2 23.7 23.9 24.1 T = 288.15 K 17.0 21.2 23.7 25.8 26.3 26.5 26.7 T = 293.15 K 20.1 24.4 26.7 28.8 29.2 29.4 29.7

⎛ ⎛1 1 − xS ⎞ 1 ⎞ ln⎜1 + λ ⎟ = λh⎜ − ⎟ xS ⎠ Tm ⎠ ⎝T ⎝

102xcal,λh S

x0B

102xexp S

9.96 13.5 16.4 18.4 19.0 19.1 19.3

0.40 0.50 0.60 0.70 0.80 0.90 1.00

23.7 27.2 29.8 31.2 32.1 32.4 33.0

12.1 15.8 18.7 20.7 21.3 21.4 21.6

0.40 0.50 0.60 0.70 0.80 0.90 1.00

27.4 31.6 33.4 35.0 36.2 36.2 36.9

14.5 18.4 21.2 23.2 23.8 24.0 24.2

0.40 0.50 0.60 0.70 0.80 0.90 1.00

34.3 36.9 38.7 40.7 41.0 41.3 41.6

17.3 21.3 23.9 26.0 26.5 26.7 27.0

0.40 0.50 0.60 0.70 0.80 0.90 1.00

38.5 42.1 43.4 45.3 45.5 45.7 45.9

24.3 28.1 30.4 32.4 32.9 33.1 33.4 28.4 32.2 34.1 36.0 36.6 36.8 37.1 33.1 36.6 38.3 40.1 40.6 40.8 41.2 38.4 41.6 42.9 44.6 45.1 45.3 45.7

X-ray Powder Diffraction Analysis. To ensure the crystal form of ethyl vanillin, the X-ray powder diffraction was used to identify the solid samples in this work. And it was found that the PXRD pattern of all samples remained consistent. The PXRD data did not show any polymorphism, solvates, or amorphous. One typical result is shown in Figure S3. It could be found that the sample has high crystallinity. Solubility Data. The experimental solubility data of ethyl vanillin in four binary solvent mixtures (water + propan-2-one/ methanol/ethanol/propan-2-ol) are listed in Table 2−Table 5 and graphically plotted in Figure 2−Figure 5. From Table 2−Table 5, the experimental mole fraction solubility data of ethyl vanillin are 0.199, 0.216, 0.237, 0.271, 0.300, 0.330, 0.369, 0.416, and 0.459 in pure propan-2-one solvent at different temperatures range from (273.15 to 313.15) K, which are consistent with the literature values.19 This confirms that the method used in this work is reliable. It can also be seen that the solubility data of ethyl vanillin increase with the increasing

(6)

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RESULTS AND DISCUSSION Melting Thermodynamics. The thermal analysis (DSC/ TG) of ethyl vanillin is shown in Figure S2. The melting temperature Tm and enthalpy of fusion ΔfusH of solute were 351.0 K and 23.6 kJ/mol, respectively, which is close to the literature value Tm = 349.8 ± 0.1 K and ΔfusH = 23.1 ± 0.2 kJ/ mol.18 The entropy of fusion ΔfusS was calculated by eq 7, and its value was found to be 67.21 J/K. ΔfusH ΔfusS

T = 298.15 K 23.7 28.0 30.0 32.1 32.6 32.7 33.1 T = 303.15 K 28.0 32.0 33.9 35.9 36.4 36.6 37.0 T = 308.15 K 33.1 36.6 38.3 40.1 40.7 40.9 41.3 T = 313.15 K 39.2 41.7 43.4 44.9 45.6 45.9 46.2

102xcal,λh S

a 0 xB is the initial mole fraction of propan-2-one in the binary solvent cal,Apel and mixture; xexp S is the experimentally determined solubility; xS cal,λh are the calculated solubility according to eq 5and eq 6, xS respectively. The standard uncertainty of T is u(T) = 0.1 K. The relative standard uncertainty of the solubility measurement is ur(x) = 0.04. The relative uncertainty of pressure is ur(p) = 0.05. The relative standard uncertainty of the solvent composition is ur (x0B) = 0.05.

20.6 24.5 27.0 29.0 29.6 29.8 30.1

where xS is the molar solubility of ethyl vanillin; Tm represents the melting temperature of solid solute; λ and h reflect the nonideality and the enthalpy of solution, respectively.17

Tm =

102xcal,Apel S

(7) C

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Table 3. Mole Fraction Solubility (xS) of Ethyl Vanillin in the Binary (Methanol + Water) Solvent Mixtures at Different Temperatures from (273.15 to 313.15) K (p = 0.1 MPa)a x0B 0.40 0.50 0.60 0.70 0.80 0.90 1.00 0.40 0.50 0.60 0.70 0.80 0.90 1.00 0.40 0.50 0.60 0.70 0.80 0.90 1.00 0.40 0.50 0.60 0.70 0.80 0.90 1.00 0.40 0.50 0.60 0.70 0.80 0.90 1.00

102xexp S

102xcal,Apel S

T = 273.15 K 0.407 0.869 2.14 3.18 4.93 6.18 8.81 T = 278.15 K 0.610 0.659 1.28 1.42 3.12 3.13 4.51 4.63 6.64 6.72 8.21 8.17 11.4 11.4 T = 283.15 K 1.09 1.06 2.46 2.29 4.29 4.52 6.41 6.57 9.03 8.98 10.3 10.6 14.7 14.4 T = 288.15 K 1.73 1.71 3.54 3.63 6.35 6.46 9.39 9.14 11.9 11.8 13.5 13.6 18.1 17.8 T = 293.15 K 2.73 2.73 5.65 5.68 9.44 9.14 12.5 12.4 15.3 15.3 17.6 17.2 21.4 21.8 0.428 0.930 2.21 3.26 4.94 6.23 8.67

102xcal,λh S

x0B

102xexp S

0.279 0.666 1.86 3.29 4.94 6.21 9.31

0.40 0.50 0.60 0.70 0.80 0.90 1.00

4.36 8.95 13.0 16.1 19.8 21.7 25.7

0.505 1.18 2.87 4.71 6.69 8.18 11.6

0.40 0.50 0.60 0.70 0.80 0.90 1.00

7.00 13.9 18.0 22.4 23.7 26.4 30.7

0.908 2.03 4.33 6.63 8.92 10.6 14.4

0.40 0.50 0.60 0.70 0.80 0.90 1.00

10.7 20.0 25.0 28.3 29.9 32.5 35.8

1.55 3.42 6.42 9.18 11.7 13.6 17.6

0.40 0.50 0.60 0.70 0.80 0.90 1.00

17.1 29.8 32.5 34.6 37.4 38.6 41.8

102xcal,Apel S T = 298.15 K 4.34 8.78 12.8 16.6 19.4 21.5 26.1 T = 303.15 K 6.88 13.4 17.8 21.7 24.4 26.5 30.9 T = 308.15 K 10.9 20.2 24.5 27.9 30.2 32.3 35.9 T = 313.15 K 17.1 30.2 33.4 35.3 36.8 38.9 41.3

102xcal,λh S 4.36 8.98 13.2 16.6 19.4 21.6 25.7 7.05 13.9 18.3 21.8 24.4 26.6 30.6 11.1 20.6 24.7 27.9 30.3 32.3 36.0 16.8 29.4 32.4 35.1 36.9 38.8 42.0

a 0 xB is the initial mole fraction of methanol in the binary solvent cal,Apel and mixture; xexp S is the experimentally determined solubility; xS cal,λh are the calculated solubility according to eq 5and eq 6, xS respectively. The standard uncertainty of T is u(T) = 0.1 K. The relative standard uncertainty of the solubility measurement is ur(x) = 0.04. The relative uncertainty of pressure is ur(p) = 0.05. The relative standard uncertainty of the solvent composition is ur (x0B) = 0.05.

2.63 5.62 9.33 12.5 15.2 17.3 21.4

temperature at constant solvent composition, which indicates that the cooling crystallization method of using these solvents for the purification of ethyl vanillin is feasible. With the increase of the initial mole fraction of methanol, ethanol, or propan-2one, the solubility data of ethyl vanillin increase. But the solubility of it in propan-2-ol and water binary solvent mixtures first increases with the increase of propan-2-ol, then shows a maximum point and then starts to decrease. This phenomenon is called cosolvency.20,21 The phenomenon might be due to the intramolecular and intermolecular interactions including polarity, hydrogen bond, van der Waals force and so on. In this work, two models were used to correlate experimental solubility data by using MATLAB program. The calculated solubility data are listed in Table 2−Table 5. The obtained model parameters are listed in Table S2−Table S3. To test the applicability and accuracy of the models used in this paper, the average relative deviation (ARD) and root-mean-square

deviation (RMSD) are also calculated and used to compare different models. Their definitions are as follows: ARD =

1 N

N

xical − xiexpt xiexpt

∑ i=1

(8)

N

RMSD =

xcal i

∑i = 1 (xical − xiexp)2 N

(9)

xexp i

where and are the calculated molar solubility values and experimental solubility values, respectively. N is the numbers of experimental points. The calculated data of ARD and RMSD are also given in Tables S2−S3. From the very small RMSDs, it can be deduced that the calculated data of ethyl vanillin are in good accordance with the values of this work in all binary solvents. Besides, it can be seen that most of the ARD values between the experimental solubility data and the correlated solubility data are lower than D

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Table 4. Mole Fraction Solubility (xS) of Ethyl Vanillin in the Binary (Ethanol + Water) Solvent Mixtures at Different Temperatures from (273.15 to 313.15) K (p = 0.1 MPa)a x0B

102xexp S

0.40 0.50 0.60 0.70 0.80 0.90 1.00

2.38 3.27 4.44 5.24 5.25 5.61 5.64

0.40 0.50 0.60 0.70 0.80 0.90 1.00

3.11 4.58 5.71 6.44 6.79 7.14 7.21

0.40 0.50 0.60 0.70 0.80 0.90 1.00

3.91 6.07 7.24 7.99 8.36 8.76 9.11

0.40 0.50 0.60 0.70 0.80 0.90 1.00

5.26 7.80 9.24 10.3 10.5 10.7 10.9

102xcal,Apel S

102xcal,λh S

x0B

102xexp S

2.33 3.33 4.39 5.13 5.24 5.60 5.65

1.55 2.74 3.90 4.61 4.82 5.04 5.16

0.40 0.50 0.60 0.70 0.80 0.90 1.00

11.0 13.9 16.1 16.8 17.8 18.1 18.3

3.06 4.46 5.69 6.50 6.69 7.03 7.14

2.38 3.96 5.32 6.14 6.40 6.66 6.81

0.40 0.50 0.60 0.70 0.80 0.90 1.00

14.7 18.9 20.1 21.5 21.6 22.0 22.4

4.07 5.96 7.36 8.23 8.50 8.83 9.00

3.61 5.62 7.17 8.08 8.39 8.69 8.87

0.40 0.50 0.60 0.70 0.80 0.90 1.00

21.6 25.4 25.6 25.9 27.2 27.9 28.3

5.50 7.96 9.50 10.4 10.8 11.1 11.3 T = 293.15 K 7.63 7.54 10.6 10.6 12.5 12.2 13.4 13.2 13.6 13.6 13.9 13.9 14.1 14.2

5.37 7.86 9.53 10.5 10.9 11.2 11.4

0.40 0.50 0.60 0.70 0.80 0.90 1.00

28.6 32.3 32.7 33.2 33.3 33.7 34.0

T = 273.15 K

T = 278.15 K

T = 283.15 K

T = 288.15 K

0.40 0.50 0.60 0.70 0.80 0.90 1.00

102xcal,Apel S

102xcal,λh S

T = 298.15 K 10.5 14.1 15.7 16.6 17.2 17.5 17.8 T = 303.15 K 14.7 18.7 20.1 21.0 21.6 21.9 22.3 T = 308.15 K 20.8 24.8 25.8 26.4 27.0 27.5 27.8 T = 313.15 K 29.8 32.7 32.9 33.3 33.8 34.4 34.7

11.2 14.6 16.2 17.1 17.6 18.0 18.3 15.6 19.3 20.6 21.4 22.0 22.4 22.7 21.3 25.1 25.9 26.6 27.1 27.6 27.9 28.3 31.9 32.1 32.5 33.1 33.5 33.9

a 0 xB xexp S

is the initial mole fraction of ethanol in the binary solvent mixture; is the experimentally determined solubility; xcal,Apel and xcal,λh are S S the calculated solubility according to eq 5 and eq 6, respectively. The standard uncertainty of T is u(T) = 0.1 K. The relative standard uncertainty of the solubility measurement is ur(x) = 0.04. The relative uncertainty of pressure is ur(p) = 0.05. The relative standard uncertainty of the solvent composition is ur (x0B) = 0.05.

7.82 10.8 12.5 13.5 13.9 14.3 14.5

heating

6.99%. It means that these models used in this work for the solubility correlation can give good correlation results. Furthermore, we take ARD values in binary (propan-2-ol + water) solvent mixtures for example. In binary (propan-2-ol + water) solvent mixtures the ARD values of the modified Apelblat equation are 1.89%, 1.53%, 0.951%, 1.05%, 1.30%, 0.989%, 0.528%, respectively, which is smaller than the ARD values of other model. The fitted curves by the modified Apelblat equation are also shown in Figures S4−S7. It shows that the modified Apelblat equation can give better correlation results and the differences between the experimental data and calculated data are very small. Dissolution Thermodynamics. For a nonideal solution, dissolution phenomenon is made up by four energetics steps based on kinetic perspective, including heating, fusion, cooling, and mixing processes. The overall dissolution process is as follows:22

solute(solid) at T ⎯⎯⎯⎯⎯⎯→ solute(solid) at Tm fusion

cooling

⎯⎯⎯⎯⎯→ solute(liquid) at Tm ⎯⎯⎯⎯⎯⎯→ solute(liquid) at T mixing

⎯⎯⎯⎯⎯→ solute(solution)atT

According to this hypothetic dissolution process, the dissolution thermodynamic can be calculated by following equations: ΔdisM = x(Δheat M + ΔfusM + Δcool M ) + Δmix M

(10)

Δmix M = ME + ΔM id

(11)

where M can be replaced by H, S, and G; x is the mole fraction solubility of ethyl vanillin; ΔfusM is the fusion thermodynamic properties; ΔheatM and ΔcoolM is the thermodynamic properties in heating and cooling process; ΔmixM is mixing thermodynamic properties of ethyl vanillin; ME and ΔmixMid represent the E

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Table 5. Mole Fraction Solubility (xS) of Ethyl Vanillin in the Binary (Propan-2-ol + Water) Solvent Mixtures at Different Temperatures from (273.15 to 313.15) K (p = 0.1 MPa)a x0B

102xexp S

0.40 0.50 0.60 0.70 0.80 0.90 1.00

3.20 3.99 4.69 4.78 4.77 4.18 3.36

0.40 0.50 0.60 0.70 0.80 0.90 1.00

4.17 5.33 6.01 6.21 6.07 5.39 4.33

0.40 0.50 0.60 0.70 0.80 0.90 1.00

5.51 7.10 7.71 7.99 7.61 6.92 5.52

0.40 0.50 0.60 0.70 0.80 0.90 1.00

7.05 8.80 9.78 10.1 9.55 8.62 7.20

102xcal,Apel S

102xcal,λh S

x0B

102xexp S

3.16 4.00 4.65 4.75 4.73 4.18 3.36

2.69 3.73 4.30 4.54 4.35 3.65 2.57

0.40 0.50 0.60 0.70 0.80 0.90 1.00

12.7 15.1 16.2 16.5 15.8 14.4 12.4

4.18 5.31 6.02 6.21 6.05 5.36 4.30

3.82 5.10 5.77 6.05 5.79 4.97 3.67

0.40 0.50 0.60 0.70 0.80 0.90 1.00

17.3 19.6 20.8 21.0 20.2 18.8 16.6

5.54 6.99 7.76 8.04 7.71 6.88 5.56

5.33 6.87 7.65 7.96 7.61 6.67 5.16

0.40 0.50 0.60 0.70 0.80 0.90 1.00

22.9 25.5 26.2 26.4 25.3 24.2 22.1

7.34 9.13 9.96 10.3 9.82 8.83 7.24 T = 293.15 K 10.2 9.72 12.2 11.9 12.8 12.7 13.6 13.2 12.8 12.5 11.5 11.3 9.57 9.5

7.34 9.15 10.0 10.4 9.89 8.85 7.14

0.40 0.50 0.60 0.70 0.80 0.90 1.00

29.0 31.1 32.0 32.4 31.1 30.4 29.4

T = 273.15 K

T = 278.15 K

T = 283.15 K

T = 288.15 K

0.40 0.50 0.60 0.70 0.80 0.90 1.00

Δheat S = Cp(s) ln

Tm T

Δcool H = Cp(l)(T − Tm) T Δcool S = Cp(l)ln Tm

T = 298.15 K 12.9 15.3 16.2 16.7 15.8 14.5 12.5 T = 303.15 K 17.0 19.6 20.5 21.0 19.9 18.7 16.6 T = 308.15 K 22.5 24.9 25.9 26.2 25.1 23.9 22.1 T = 313.15 K 29.6 31.6 32.5 32.5 31.5 30.7 29.6

102xcal,λh S 13.3 15.6 16.5 16.9 16.1 15.0 13.1 17.5 19.9 20.8 21.2 20.3 19.1 17.3 22.7 25.1 26.0 26.3 25.2 24.1 22.4 28.8 31.1 31.9 32.2 30.9 30.0 28.6

a 0 xB is the initial mole fraction of propan-2-ol in the binary solvent cal,Apel and mixture; xexp S is the experimentally determined solubility; xS cal,λh are the calculated solubility according to eq 5 and eq 6, xS respectively. The standard uncertainty of T is u(T) = 0.1 K. The relative standard uncertainty of the solubility measurement is ur(x) = 0.04. The relative uncertainty of pressure is ur(p) = 0.05. The relative standard uncertainty of the solvent composition is ur (x0B) = 0.05.

9.96 12.0 12.9 13.3 12.7 11.6 9.74

properties of ideal systems can be calculated by following equations:

excess properties and mixing properties of ideal systems, respectively. Thermodynamic properties in heating and cooling process can be calculated by the following equations: Δheat H = Cp(s)(Tm − T )

102xcal,Apel S

n

Δmix Gid = RT ∑ xi ln xi i

(12)

Δmix H id = 0 (13)

(16) (17)

n

Δmix S id = −R ∑ xi ln xi

(14)

i

(18)

where xi is the mole fraction of component i in real solution. In binary solution, n = 2. The excess mixing properties can be calculated as follows:23

(15)

Compared with the thermodynamic property of fusion, the item of (ΔheatM + ΔcoolM) is much smaller and can be ignored. Because the system is in equilibrium during the phase transition of solute, the item of ΔfusG is zero. In addition, mixing

n

GE = RT ∑ xi ln γi i

F

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Figure 2. Mole fraction solubility of ethyl vanillin depending on temperature T and the mole fraction of propan-2-one (xS) in the binary (propan-2-one + water) solvent mixtures.

Figure 4. Mole fraction solubility of ethyl vanillin depending on temperature T and the mole fraction of ethanol (xS) in the binary (ethanol + water) solvent mixtures.

Figure 3. Mole fraction solubility of ethyl vanillin depending on temperature T and the mole fraction of methanol (xS) in the binary (methanol + water) solvent mixtures. n ⎛ ∂ ln γi ⎞ HE = −RT 2 ∑ xi⎜ ⎟ ⎝ ∂T ⎠ px i

HE − GE SE = T

Figure 5. Mole fraction solubility of ethyl vanillin depending on temperature T and the mole fraction of propan-2-ol (xS) in the binary (propan-2-ol + water) solvent mixtures.

Gij = exp( −αijτij)

(23)

(20)

τij = (21)

gij − gii RT

= Δgij /RT

(24)

where Δgij is parameter in this model and it stands for the intermolecular interaction energy; τ is a constant representing the nonrandomness of the solvent; α is an adjustable value which can be chosen according to the specific situation. The dissolution thermodynamic properties of ethyl vanillin calculated are shown in Tables S4−S7. From these tables, it can be seen that the values of ΔdisG are all negative, which proves that the dissolution process of ethyl vanillin is spontaneous. The values of ΔdisS are all positive, which means that the dissolution process of ethyl vanillin is entropy-driven. The values of ΔdisH are all positive, which means that the dissolution process of ethyl vanillin is endothermic.

where γi is the activity coefficient of component i in real solution; the meaning of other symbols are the same with previous equation. Furthermore, the activity coefficient can be calculated by the NRTL model.24,25 ln γi = (Gjixj + Gkixk)(τjiGjixj + τkiGkixk)(xi + xiGjixj + xkGki)2 + [τijGijxj2 + GijGkjxjxk(τij − τkj)]/(xj + xiGij + xkGkj)2 + [τikGik xk2 + Gik Gjk xjxk(τik − τjk)]/(xk + xiGik + xjGjk )2 (22)

where Gij, Gik, Gji, Gjk, Gki, Gkj and τij, τik, τji, τjk, τki, τkj are model parameters. Here, these terms can be expressed as G

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CONCLUSIONS In this work, the solubility of ethyl vanillin in four kinds of binary solvent mixtures (water + propan-2-one/methanol/ ethanol/propan-2-ol) were measured at temperatures ranging from 273.15 to 313.15 K by using the UV method. In the selected solvent mixtures, the solubility of ethyl vanillin increases with increasing temperature. With the increase of the initial mole fraction of methanol, ethanol, or propan-2-one, the solubility data of ethyl vanillin increase. However, the solubility of ethyl vanillin shows a maximum point with the increase of the initial mole fraction of propan-2-ol. Besides, the correlated values based on the modified Apelblat equation and the λh model show good agreement with the experimental values. Finally, the dissolution thermodynamic of ethyl vanillin, including enthalpy, entropy, and Gibbs energy, was calculated based on the NRTL model. According to the data, it was found that the dissolution process of ethyl vanillin is spontaneous, entropy-driven, and endothermic.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.6b00972. Comparison of solubility data for protocatechuic acid in methanol between experimental values obtained in this work and literature data;8 parameters of different models for ethyl vanillin; dissolution thermodynamic properties of ethyl vanillin in different binary solvent mixtures; thermal analysis (TGA/DSC) of ethyl vanillin; X-ray powder diffraction pattern of ethyl vanillin; fitted curves by the modified Apelblat equation (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86-22-27405754. Fax: +86-22-27374971. ORCID

Hongxun Hao: 0000-0001-6445-7737 Funding

This research is financially supported by National Natural Science Foundation of China (No. 51478308) and Major National Scientific Instrument Development Project (No. 21527812). Notes

The authors declare no competing financial interest.

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